Anesthesia delivery machines provide breathing gases, such as air, oxygen, helium, and nitric oxide, along with anesthetic agents to a patient during surgery or diagnostic procedures. A recirculating breathing circuit returns expired breathing gases back to the patient. A recirculating breathing circuit conserves anesthetic agent and lowers the overall cost of a medical procedure. The use of a recirculating breathing circuit also provides the additional advantage of conserving heat and moisture in the breathing gases, thereby avoiding discomfort to the patient.
One problem that must be overcome in a recirculating breathing circuit is the removal of carbon dioxide (CO2), a byproduct of cellular metabolism, from the expired breathing gases before they can be delivered back to the patient. This is performed by a CO2 absorber canister in the breathing circuit. The canister is filled with a CO2 absorbent material that removes the CO2 from the expired gases as they pass through the canister. Two common types of CO2 absorbent materials are soda lime and baralyme although there are other chemicals that may serve this purpose. The CO2 absorbent materials may include a color changing indicator that informs a clinician when the CO2 absorbing reagents have been used up and the CO2 absorbent material must be replaced. U.S. Pat. No. 3,088,810 to Hay discloses a transparent CO2 absorber canister utilizing a CO2 absorbent material that is treated with an indicator agent that, during the course of CO2 absorption, indicates the depletion of the absorptive capacity of the material by gradually changing color. Ethyl violet is disclosed as being one such indicator which changes color from white to blue/purple as the absorptive capacity of the absorbent becomes exhausted. U.S. Pat. No. 5,360,002 to Smith discloses a similar CO2 absorbent material in a disposable canister.
The CO2 absorbent material found in the canister of a recirculating breathing system typically contains about 20% water by weight. This hydration helps to initiate the reaction that removes the CO2 gas from expired breathing gases passing through the canister. However, as noted below, the potential exists in a CO2 absorber canister for the CO2 absorbent material to dry out before the CO2 reagents have been expended. The water content of a CO2 absorbent material considered to be “dried out” varies among CO2 absorbent materials. While soda lime is considered to be dried out at a water concentration less than approximately 2% water by weight, baralyme is considered to be dried out at concentrations less than approximately 5% water by weight. At these water concentrations, the CO2 absorbent reaction is still maintained.
However, the anesthetic agent in the expired breathing gases will react with the dried out CO2 absorbent material in an exothermic reaction which produces excess heat and toxic substances within the breathing circuit. The type of reaction that takes place in the absorber canister, as well as the products of the anesthetic agent/dried out CO2 absorbent material reaction, is dependent on the type of anesthetic agent in use. Anesthetic agents such as desflurane, enflurane, and isoflurane react with dried CO2 absorbent material to produce carbon monoxide. Carbon monoxide is dangerous because of the toxic effects it has on the body, a danger that is exacerbated with sick or surgical patients. Other anesthetic agents react with dried CO2 absorbent material to form other toxic compounds. Halothane reacts with CO2 absorbent material to form a substance called BCDFE (2-bromo-2-chloro-1,1-difluoroethylene) while, in the case of sevoflurane, a known nephrotoxin referred to as Compound A (fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether) is formed. Additionally, the chemical reaction that produces Compound A is highly exothermic and the increased heat speeds up the reaction, thus creating more Compound A and a potential fire hazard.
While the potential for the existence of these adverse circumstances has always been present in recirculating breathing systems, these reactions only truly began to pose a potential threat to patients with the recent increased focus on and use of low-flow and minimal-flow anesthesia techniques. Due to economical constraints, newer low-flow and minimal-flow anesthesia systems are designed to have more efficient breathing circuits that recycle as much medical gas as possible and use lower flow rates of breathing gas, thus using less anesthetic agent and medical gas and reducing the overall cost of anesthetization, as is taught by Keitel et. al. in U.S. Pat. No. 6,216,690. The increased recirculation of the breathing gases and the lower gas flow rates result in greater overall concentrations of toxic gases produced as byproducts of undesired reactions within the patient breathing circuit that were not seen before in less efficient systems.
The CO2 absorbent material in the canister may become dried out, thus producing toxic byproducts, in a variety of ways. The largest contributor is the breathing gases themselves. Breathing gases are supplied from tanks or manifolds in which the gas is compressed. During compression of the gases, water from the gases is removed and, as such, they are extremely dry when the pressure is lowered and the gases are supplied to the patient. While breathing gases expired by the patient have a humidity near 100%, standard ventilation procedures utilize a small, continuous flow of breathing gas through the breathing circuit, known as a bias flow. This bias flow reduces the patient's airway resistance, reducing the effort required to carry out respiration as well as increasing the overall efficiency of the mechanical ventilation that accompanies anesthesia. This bias flow will constantly place a quantity of dry medical gas in contact with the CO2 absorbent material, thus promoting drying out the CO2 absorbent material, notwithstanding the humidity of the breathing gases expired by the patient. This drying out is increased during periods of high gas flow, such as the inductance and emergence phases of anesthesia, when the bias flow is supplemented with increased amounts of fresh gas.
CO2 absorbent material may also become dried out as a result of nonstandardized hospital procedure in shutting down and starting up anesthetic delivery apparatus. This can result in failure to completely close the valves on the medical gas supply tanks or manifolds. This may result in a small flow of dried medical gas coming in contact with the CO2 absorbent material over an extended period of time, such as the course of a night or weekend. The existence of this problem is supported by medical data showing that patient exposure to toxic substances formed because of dried CO2 absorbent material are most likely to occur during the first procedure performed at the start of the week.
The final factor contributing to this problem is the common hospital practice of reusing CO2 absorbent material until its CO2 absorbent properties have been spent. This prolongs the time in which a CO2 absorbent material may be exposed to medical gases in a sufficient quantity to dry the CO2 absorbent material, thus creating the aforementioned hazardous conditions.
It would, therefore, be desirable to provide a breathing circuit CO2 absorber canister with means to indicate the potential for hazardous conditions, such as the generation of heat and/or toxic gases. Preferably such an indicator should be provided in a manner that avoids interference with the action of the CO2 absorbent depletion indicator.
Color changing moisture indicators utilizing cobalt chloride (CoCl2) have been in use for some time and are well known. U.S. Pat. No. 5,224,373 to Williams discloses a container with a flexible humidity indicator using such a chemical and capable of displaying information about the humidity of the environment inside the container. U.S. Pat. No. 4,150,570 to Fuller shows a humidity sensing device for visually indicating changes in relative humidity as by numbers or text.
However, such prior art does not specifically address the problem of monitoring hazardous heat and toxic gas generating conditions within a recirculating breathing system CO2 absorber canister.